In this age of increased air pollution, the removal of chemicals from the air we breathe is a concern of everyone. In addition, in the fabrication electronic materials and of devices such as semiconductors, there is a requirement for uncontaminated air of high quality.
Gas phase filtration is commonly accomplished using activated carbon manufactured in various ways. One approach uses a carbon/adhesive slurry to glue the carbon to the substrate. The adhesive decreases carbon performance by forming a film on its surface.
A second approach involves carbonizing an organic based web by heating, followed by carbon activation. This material is expensive and has relatively low adsorption capacity.
A third approach involves forming a slurry of carbon powders and fibers into sheets by a process analogous to a wet papermaking process. This material has a medium-to-high cost, and has an undesirable high pressure drop. Moreover, chemically impregnated carbon particles cannot be efficiently used in conjunction with an aqueous process, as the aqueous nature of the process either washes away the chemical used to impregnate the carbon, or reacts undesirably with the impregnating or active chemical groups thereby rendering it inoperative. In general, however, filter materials which do not incorporate chemically active groups perform far less effectively than those which do include chemically active groups.
None of these approaches fully achieve the desired properties which provide a clean, cost effective, high efficiency, low pressure drop, adsorptive composite.
The present invention comprises a clean, cost effective, high efficiency, low pressure drop, adsorptive, filter comprising a high surface area, highly acidic chemically impregnated adsorbent. A preferred embodiment of the invention can employ a non-woven composite material having acidic functional groups that bind to airborne bases. The invention can be used in lithography systems which employ materials that are sensitive to impurities, such as molecular bases present in the air circulating through semiconductor wafer processing equipment. A large number of bases including ammonia, NMP, triethylamine pyridine, and others, can be maintained at concentrations below 2 ppb in a tool cluster filtered with the present invention. The invention also includes methods for forming the filter comprising, for example, the dry application of an active adsorbent to a non-woven carrier material, which is then heated and calendered with cover sheets.
In a preferred embodiment of this invention the non-woven carrier materials can be polyester non-wovens, and the adsorbent can include sulfonated divinyl benzene styrene copolymer. Another preferred embodiment employs carboxylic functional groups. The acidic groups have at least 1 milliequivalent/gram acidity level or higher and preferably at least 4.0 milliequivalents/gram or higher. The polymers used are porous, preferably having a pore size in the range of 50-400 angstroms and a surface area of 20 m2/g or higher.
The dry processing of a non-woven polyester batting allows for even distribution of adsorbent particles throughout the depth of the polyester batting. This provides an increased bed depth at a very low pressure drop, which is highly desirable since a twofold increase in bed depth can increase the filter""s breakthrough time (time to failure) fourfold when using these thin fabric based sulfonic beds.
Thus, the invention provides a clean, cost effective, high efficiency, low pressure drop, adsorptive non-woven filter composite, and a method for forming said composite. The novel filter composite is particularly useful for the removal of base contaminants in an air stream, which contaminants can be gaseous or vaporous. Particulates will also be removed if greater than the pore size of the filter. The filter has a service life of at least 12 months with a pressure drop to reduce power consumption and minimize impact on the systems operation. For example, a high pressure drop filter can require a longer time for a lithography system to equilibrate the temperature and humidity after filter replacement.
In an alternative embodiment, the filter uses both sulfonated divinyl benzene styrene copolymer and an activated carbon as the adsorbent. Activated carbon is discussed in greater detail in U.S. Pat. No. 5,582,865 titled xe2x80x9cNon-Woven Filter Composite.xe2x80x9d The entire contents of this patent is incorporated herein by reference. The filter in this preferred embodiment has two (or more) layers, one of activated carbon and one of sulfonated divinyl benzene styrene copolymer beads. Additionally, two or more materials can be mixed to provide the filter system of the present invention.
In another preferred embodiment a synthetic carbon material, such as that described in U.S. Pat. No. 5,834,114, the contents of which are incorporated herein by reference, can be coated with the materials of the present invention to provide a porous acidic filter in accordance with the invention.
A detection system and method of use for determining when the filter needs to be replaced by detecting base contaminants in air is described in U.S. patent application Ser. No. 09/232,199 entitled xe2x80x9cDetection of Base Contaminants in Gas Samplesxe2x80x9d, filed on Jan. 14, 1999 now U.S. Pat. No. 6,207,460 with Oleg Kishkovich, et al as inventors. Also U.S. patent application Ser. No. 08/795,949 entitled xe2x80x9cDetecting of Base Contaminantsxe2x80x9d, filed Feb. 28, 1997 now U.S. Pat. No. 6,096,267 with Oleg Kishkovich, et al as inventors and U.S. patent application Ser. No. 08/996,790 entitled xe2x80x9cProtection of Semiconductor Fabrication and Similar Sensitive Processesxe2x80x9d, filed Dec. 23, 1997 now U.S. Pat. No. 6,296,806 with Oleg Kishkovich, et al as inventors can also be used with the present invention. These patent applications disclose the protection of a DUV lithography processes using chemically amplified photoresists which are sensitive to amines in the air. These patent applications are incorporated in the present application in their entirety by reference.
A preferred embodiment of a method of fabricating a filter element having a large surface area and the desired flow characteristics involves the use of a powdered material that is deposited in sequential layers one on top of the other. Following the deposit of each layer of powdered material, a binder material is delivered onto each layer of powdered material using a printing technique in accordance with a computer model of the three dimensional filter element being formed. Following the sequential application of all of the required powder layers and binder material to form the part in question, the unbound powder is appropriately removed, resulting in the formation of the desired three dimensional filter element. This technique provides for the fabrication of complex unitary or composite filter elements having high surface area that are formed with a very high degree of resolution.